In de-salting (desalination) systems (e.g., temperature-induced, pressure-induced, or a combination), source water to be de-salted is generally unsaturated of some scaling compounds (e.g., gypsum in seawater; Table 1: S1). The inventor illustrates this situation in FIG. 1. During de-salting, source water is brought closer to saturation of such scaling compounds, but simultaneously, the concentrations of the background salts are also increasing. Since the solubilities of the scaling compounds vary with the concentrations of the background salts, the solubilities of the scaling compounds are constantly changing as de-salting continues. As such, the entire history of de-salting has been evolving around avoiding scale formation by operating de-salting systems within the thresholds (solubility limits) of the scaling compounds that change with changing the concentrations of the background salts. However, neither scaling thresholds in an actual de-salting system can be inferred from simple solubility measurements nor can be avoided by conventional source water pre-treatment (e.g., by adding an acid to control alkaline scale and a scale inhibitor to delay sulfate scale). As a result, de-salting systems cannot be brought to their intrinsic productivities, and their brine disposal presents a problem in terms of volume and scale-infested species (e.g., Table 1: S2 to S4).
Such, is illustrated, for example, in two recently installed conventional recycle-brine multi-stage flash (RB-MSF) desalination systems (plants) that produce about 240 million U.S. gallons per day (MGD) of distillate from seawater. Each plant comprises 8 identical and independent RB-MSF desalination trains. An RB-MSF desalination train is illustrated in FIG. 2 (application Ser. No. 15/731,626 and application Ser. No. 15/731,637; FIG. 6, Configuration B, an RB-MSF desalination train). The needed amount of total seawater feed stream for both plants is 2,247 MGD, which is enormous. The total seawater feed stream comprises two portions: (1) about 1538.2 MGD as cooling seawater for the heat rejection sections of the RB-MSF desalination trains that must be partially pre-treated (e.g., screening and chlorination) and rejected back to a sea; and (2) about 654.8 MGD as the actual seawater feed stream to produce distillate, which is conventionally pre-treated (e.g., by de-carbonating and adding scale inhibitors, oxygen scavengers and foam suppressors). Here, the latent heat from the evaporating vapor in the heat rejection sections of the RB-MSF desalination trains is exhausted to waste in reject cooling seawater. Yet, the ratio of distillate to total seawater feed stream is about 10%, which is an unmatched inefficiency in terms of distillate recovery ratio, the required enormous pumping power for the total seawater feed stream and reject cooling seawater, and relatively high conventional seawater pre-treatment costs.
The amount of recycle brine in both plants that must be constantly circulated through the heat recovery sections of the RB-MSF desalination trains is 2,185.8 MGD, which is also enormous and nearly equivalent to the amount of the total seawater feed stream. Similarly, it requires an enormous pumping power as well as treatment with scale inhibitors and oxygen scavengers.
The amount of heavily scale-infested reject brine (Table 1: S3) from both plants is 905.4 MGD, which is also enormous and must be blown down to a sea. Such reject brine: (1) increases salinity, including all of the concentrated scale pairing ions (magnesium, calcium and sulfate), around seawater intake lines, which, in turn, deteriorates the natural ions composition of seawater (changes the thresholds of scaling compounds) and imposes different sets of operating conditions on the plants; and (2) environmentally impacts marine habitats since reject brine is depleted of oxygen as well as enriched with residues of deoxygenating agents, concentrated toxic species (e.g., derivatives of boron and chlorine), and gypsum.
The scale formation problem has taken on new proportions arising from the urgent need to de-salt source water of saturated scaling compounds, high oil content, high salinity, or a combination (e.g., produced water and the like). Wet oil, which is a macro-emulsion, is the source of produced water. Since macro-emulsions are not thermodynamically stable, they will naturally segregate into the original phases (oil and water), if given enough (may be infinite) time to rest. In wet oil processing; however, the needed time to segregate oil from water is transcended by a gravity separator (e.g., a two- or three-phase separator, a gravity tank, a skim tank, etc.), an accelerated separator (a hydrocyclone or a centrifuge), or a combination. Such wet oil segregation processes are basically breaking down a given “primary” emulsion, whether the dispersed phase in the “primary” emulsion is water (a “water-in-oil” emulsion) or the dispersed phase is oil (an “oil-in-water” emulsion), into two “secondary” emulsions; one is richer and the other is poorer in the dispersed phase of the “primary” emulsion. The inventor illustrates such wet oil segregation approaches in FIG. 3. As such, neither the water phase is sufficiently de-oiled nor is the oil phase sufficiently de-watered; thereby each of the phases (oil and water) requires further multiple and intricate processing steps. Yet, charged organic species (e.g., oxygen-, nitrogen-, and sulfur-containing species) are common in wet oil, which not only hinder the segregation of the oil phase from the water phase but also heavily contribute to scale formation in the water phase. Yet, as illustrated in FIG. 4, for example, there may be no thresholds (e.g., the solubility of gypsum is at the saturation limit) of the scaling compounds in produced water to “zero in” for at least partially operating a de-salting method. Thus, a de-salting method may be forced to operate at, or above, the saturation limits of scaling compounds.
Such, is also illustrated, for example, in FIG. 5 wherein produced water may be de-salted by a mechanical vapor recompression (MVR) system without proper obviation of the oil content and saturated scaling compounds. Here, the de-oiling steps are deficient since they generate roughly de-oiled produced water that carries over at least some of the oil content to the feed heat exchanger, steam stripper and MVR (evaporator). The carried over oil content acts as a foulant for heat transfer surfaces and causes severe foaming problems.
Produced water may be also saturated with calcium sulfate (gypsum) before processing (e.g., FIG. 4; Table 1: S6 and S7). As evaporation progresses in the feed heat exchanger and the steam stripper prior to the evaporator, calcium sulfate along with other notorious scaling compounds are increasing in concentrations, which would: (1) cause scale fouling/plugging problems; (2) reduce heat transfer efficiency; and (3) elevate the boiling point thereby reducing the temperature driving force for heat transfer. The latter is a critical factor in designing a conventional MVR with low temperature driving force above normal boiling of a saline stream.
Aside from the feed heat exchanger and steam stripper that are directly subjected to calcium sulfate scaling, such scaling is presumably controlled within only the evaporator by a seeding mechanism. Since the main scaling compound (gypsum) in produced water is at it is saturation limit, a sulfate-based compound (e.g., sodium sulfate or calcium sulfate) is used as a seeding agent in the evaporator to presumably minimize tubes plugging. However, hemihydrate is the first form of calcium sulfate hydrates to precipitate in the evaporator according to the rule of “stepwise sequence” of phase transformations (from less stable to more stable forms) and it is precipitation evolves rapidly and for a relatively finite time (e.g., extends to several hours) compared to the detention time elapsing during the circulation of brine through the evaporator. Thus, the metastable hemihydrate would continuously deposit on the heat transfer tubes even though calcium sulfate is readily supersaturated in the slurry but the anhydrite stable form may not be attained quickly enough to minimize tubes plugging
The seeding agent must be selected of the same form that deposits during evaporation but even if a selected form of calcium sulfate was used as a seeding agent, different forms of calcium sulfate (hemihydrate and anhydrite) would co-exist and vary with the conditions in the evaporator. If sodium sulfate was used as a seeding agent, on the other hand, the forms of sodium sulfate would have a temperature-solubility phase diagram [see e.g., U.S. Pat. Nos. 7,501,065 and 8,197,696] that totally differs from the temperature-solubility phase diagram of calcium sulfate forms. In addition, the seeding agent must be dispersed in the evaporator in the form of very fine particles, and the amount of the seeding agent must substantially exceed the concentration of calcium sulfate in produced water. Thus, the seeding mechanism is very difficult to control since the: (1) seeding agent may be a mismatch (in terms of type, form, particle size, and combinations of these factors) even though it is in the form of sulfate; and (2) amount of the seeding agent is considerable. As a result, the seeding mechanism: (1) requires a high flow rate to evaporate produced water in the heat transfer tubes, which may diminish the evaporation efficiency; and (2) is not adoptable in multi-stage flash evaporators wherein the boiling point of circulated brine is successively reduced by reducing pressure.
As such, the conventional primitive management of scale problems vary from operating a de-salting system within the thresholds (solubility limits) of the scaling compounds that change with changing the concentrations of the background salts (e.g., seawater), to operating a de-salting system from saturation to induced supersaturation of scaling compounds (e.g., some produced water). As a result, scale problems remain the focal issue that historically diminishes the productivity of any de-salting system.
The inventor consistently characterizes such reactive scale avoiding and scale overriding approaches (operating within the thresholds of scaling compounds such as calcium sulfate and using scale inhibitors or operating above the thresholds of scaling compounds such as calcium sulfate and using seeding mechanisms as well as scale inhibitors) as severely deficient to solve scale problems, whether source water is used for feeding a de-salting system, oil-gas fields' water applications, or the like. However, the inventor rather teaches proactive approaches by not only “zeroing in” on selectively and effectively removing scale prone compounds but also on recovering such compounds as useful by-products, thereby allowing any de-salting system, for example, to reach its intrinsic productivity in the absent of scale [e.g., U.S. Pat. Nos. 6,365,051; 6,663,778; 7,093,663; 7,392,848; 7,501,065; 7,789,159; 8,915,301; 9,701,558; application Ser. No. 15/731,626; and application Ser. No. 15/731,637].
Similarly, the inventor consistently characterizes wet oil processing centers as a dual problem since neither the oil phase is sufficiently recovered (de-watered) nor is the water phase (produced water) sufficiently de-oiled, and yet there are still the questions of the: (1) disposal of oily waste streams, oily stripping streams, exhausted adsorption materials, or a combination; (2) environmental impact of discharging produced water; and (3) beneficial use of produced water by de-salting methods remains hindered since neither efficient nor economic de-salting methods can be operated in the absent of efficient de-oiling as well as de-scaling. The inventor, instead, teaches that water de-oiling and oil de-watering are synonymous, and thus they should be simultaneously targeted by an efficient method. The inventor's de-oiling/de-watering concept [e.g., U.S. Pat. Nos. 6,365,051; 7,789,159; 7,934,551; 7,963,338; 8,915,301; 9,701,558; application Ser. No. 15/731,626; and application Ser. No. 15/731,637] is analogous to the natural demulsification phenomenon (a capillary flow) of oil in downhole reservoirs. The inventor illustrates the concept in FIG. 6, whether oil is the continuous phase (a “water-in-oil” emulsion) or oil is the dispersed phase (an “oil-in-water” emulsion). Here, by utilizing the hydrophobic interactions between oil and water (immiscible fluids) along with a properly configured hydrophobic membrane, water (the membrane's non-wetting fluid) would be efficiently repelled while oil (the membrane's wetting fluid) would be permeated through the hydrophobic membrane by applying a low pressure.
As re-emphasized above, de-scaling, de-oiling or a combination is critical since scale, oil content or a combination diminishes the productivity of any de-salting method. De-scaling, in particular, is also very critical since the disposal of reject brine from any de-salting method presents a problem. As such, minimizing the volume of reject brine, if not directly utilizing reject brine, is highly desirable, but is not attainable without effective de-scaling as well as effective de-salting. For effective de-salting, the combination of the multistage flash principle with the vapor compression principle is also highly desirable, but to this day, such a combination has not been attained in a practical manner.